Enhancement of transient current has been investigated in MIS (p) structure by thinning part of gate metal thickness [1]. To acquire further understanding of the transient phenomenon in C-V characteristic, we use forward and backward C-V hysteresis profiling to observe the delayed carriers’ response due to thin metal gate induced voltage drop. By forward and backward voltage sweeping, two capacitance states can be easily induced and the hysteresis of the C-V characteristic can become noticeable.

The schematic cross section and top view of our device is shown in Fig. 1. The MIS structure with elongated thin metal gate (ETGMIS) is formed by thin Al film circles in concentric with the thick Al gate, in which the inner radius R₁ is 122.5um and the outer radius R₂ is 142.5um, and the thickness of thick and thin Al films are around 250nm and 10nm, respectively. For comparison, the conventional MIS is fabricated by thick Al gate with a radius R₂ of 142.5um. From the ratio of accumulation capacitance shown in Fig. 2, it can be inferred the elongated part of metal gate does not contribute to the accumulation capacitance, which indicates the existing resistivity of the thin metal, and therefore induces transient two-states characteristics as mentioned in previous study [1].

Fig. 3 shows the C-V profiling with various frequencies for the ETGMIS and the MIS. It can be seen though C-V curve of the MIS also splits during forward and backward voltage sweeping, two-states characteristic of capacitance is noticeably enlarged by the ETGMIS, especially during low frequency measurement. It’s worth mentioning that due to the nature of carriers’ generation and recombination, the capacitance of backward voltage sweeping will generally be higher than forward voltage sweeping at the same voltage. In addition, the capacitance during forward and backward voltage sweeping starts to split after the device turn into inversion, which implies the two-states phenomenon is attributed to the minority carriers. To further illustrate this enhanced two-states phenomenon, several pulse cycles are imposed to measure the difference of capacitance at a defined read voltage, called ”capacitance window”. The applied pulse voltages are +2V and -2V to initiate the transient two–states characteristic, and the measurement is practiced at a frequency of 1kHz. Moreover, in order to obtain significant capacitance window, we define a parameter ”capacitance window ratio”, which is defined as (C_backward-C_forward)/C_backward in the C-V curve. Follow the above definition, we choose the voltage that owns the biggest capacitance window ratio as the defined read voltage after the pulse, which is about -0.2V. As depicted in Fig. 4, the capacitance window in ETGMIS can be about an order larger than in MIS.

From another point of view, it can be seen in Fig. 5 that there are obvious voltage shift between forward and backward C-V sweeps in ETGMIS than in MIS with the same EOT. To well compare the hysteresis in the C-V curve for both structures, we define the corresponding voltage as the voltage when the capacitance is 10% larger than the minimum capacitance C_min. Therefore, the hysteresis can be defined as the difference between two corresponding voltages. Apparently, as shown in Table 1, the ETGMIS owns larger hysteresis than the MIS. It is supposed that the elongated part of gate may trigger more minority carriers to late response due to its resistive nature.

Take further consideration into the delayed property, we carry out different pace of the C-V sweeping rate to observe the following hysteresis variation. As shown in Fig. 6, the hysteresis of the ETGMIS decreases as the C-V sweeping rate decreases, while the MIS displays almost insignificant variation of the C-V voltage shift. Consequently, we can reasonably suspect the C-V hysteresis enhancement is mainly attributed to the delayed characteristic of the elongated thin metal.

At last, we analyze the variation of the oxide thickness to the hysteresis in ETGMIS. In previous work [1], it is noticed the fringing field is restricted by the surrounded thin metal due to its resistive characteristic. In Fig. 7, when the oxide is thicker, the vertical electric field will become weaker. Hence, the fringing field is therefore restricted more slightly. As a result, this ETGMIS becomes more similar to the MIS; that is, tends to hold a smaller C-V hysteresis, and vice versa.

This work was supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. MOST 105-2221-E-002-180-MY3 and MOST 106-2221-E-002-196-MY3.

[1] Kuan-Hao Tseng, Chien-Shun Liao, and Jenn-Gwo Hwu, IEEE Trans. On Nanotechnology, Vol. 16, No. 6, pp. 1011-1015, Nov. 2017.